linux-kernel - Re: [patch 4/4] prctl: PR_SET_MM -- Introduce PR_SET_MM

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Date:	Sat, 23 Aug 2014 00:38:09 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org, keescook@...omium.org, tj@...nel.org,
	avagin@...nvz.org, ebiederm@...ssion.com, hpa@...or.com,
	serge.hallyn@...onical.com, xemul@...allels.com,
	segoon@...nwall.com, kamezawa.hiroyu@...fujitsu.com,
	mtk.manpages@...il.com, jln@...gle.com
Subject: Re: [patch 4/4] prctl: PR_SET_MM -- Introduce PR_SET_MM_MAP
 operation, v3

On Thu, Aug 21, 2014 at 11:49:12PM -0700, Andrew Morton wrote:
> > > 
> > > But it's a bit hacky.  Can anyone think of anything smarter?
> > 
> > Looks good to me and not that hacky actually.
> 
> Hacky :( I guess it's pretty safe because this is a userspace-visible
> structure so we'll never be changing it.

Well, I saw something similar in netfilter code a long ago :)

> 
> Or will we?  What happens if we later decide that some additional field
> needs to be added?  Do we version the interface?  Add a new prctl()
> mode?  Let's cook up a plan for that and at least add to changelog?

I don't expect to change it anytime soon but we still have an option --
if we decide to extend or shrink it we always can use sizeof/offsetof
helpers to check which exactly version userspace asks us to use.
As far as I understand the mm_struct is not the structure which
changes that frequently, right?

> > Should I update on top for -mm tree?
> 
> Spose so.  Let's see what the code savings are when the other two sites
> are similarly changed?
> 
> To save a bit more space offsets[] could be an array of uchar, I guess.
> A BUILD_BUG_ON(sizeof(struct prctl_map) >= 256) would keep that sane.

Sure, thanks!
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